The following sections describe the structural components that enable smooth muscle to set or alter hollow organ volume. These components include contractile and regulatory proteins, force-transmitting systems such as the •Fig. 14.2 Scanningelectronmicrographsofsmoothmuscle.A,Musculararteriolewithfusiformsmoothmusclecellsinacircularorientation(bar, 20µm).B,Superimposedimagesofcircular(below) andlongitudinal(above) layersofintestinalsmoothmusclesandwichingneuralcomponentsofthemyentericplexus(asterisk) (bar, 50mm).C,Rectangularsmoothmusclecellswiththinprojectionstoadjacentcellsinasmalltesticularduct(bar, 5µm).(FromMottaPM[ed].Ultrastructure of Smooth Muscle. Norwell,MA:KluwerAcademic;1990.) cytoskeleton, linkages between cells and the extracellular matrix, and membrane systems that transduce extracellular signals into changes in myoplasmic [Ca++].
A variety of specialized contacts exists between smooth muscle cells. Such contacts allow mechanical linkage and communication between cells. In contrast to skeletal muscle cells, which are normally attached at either end to a tendon, smooth (and cardiac) muscle cells are connected to each other. Because smooth muscle cells are anatomically arranged in series, they not only must be mechanically linked but must also be activated simultaneously and to the same degree. This mechanical and functional linkage is crucial to smooth muscle function. If such linkage did
Dense bodies c Caveolae Thick filaments Mitochondria Sarcoplasmic reticulum Thick Thin Filaments 0.1 µm0.1 µm C 0.5 mµ not exist, contraction in one region would simply stretch another region without a substantial decrease in radius or increase in pressure. The mechanical connections are provided by attachments to sheaths of connective tissue and by specific junctions between muscle cells.
Several types of junctions are found in smooth muscle. Functional linkage of the cells is provided by gap junctions (
Fig. 14.3A ). Gap junctions form low-resistance pathways between cells (see ). They also allow chemical communication by diffusion of low-molecular-weight compounds. In certain tissues, such as the outer longitudinal layer of smooth muscle in the intestine, large numbers of such junctions exist. Action potentials are readily propagated from cell to cell through such tissues.